Nudibranch Vanuatu

The 3,000-plus known nudibranch species are possibly the most colourful creatures on earth. In the course of evolution, these “sea slugs” have lost their shell developing other defense mechanisms. As members of the gastropod class, and more broadly the molluscs, they live fully exposed at virtually all depths of salt water, their gills forming tufts on their backs. (Nudibranch comes from the Latin nudus, naked, and the Greek brankhia, gills. "Naked gill" is a feature that separates them from other sea slugs). Varying in adult size from just 20 to 600mm, they reach their greatest size and variation in warm, shallow waters and are found extensively on Vanuatu’s many reefs.

Nudibranchs are blind to their own beauty, their tiny eyes discerning little more than light and dark. Instead the animals smell, taste and feel via oral tentacles. Chemical signals help them track food—not just coral and sponges but barnacles, eggs, or small fish - and other nudibranchs. Hermaphroditic, nudibranchs have both male and female organs and can fertilise oneanother, an ability that speeds the search for mates and doubles reproductive success. Nudibranchs are carnivorous. Some feed on sponges, others on coral animals (hydroids) and some are cannibals, eating other sea slugs, or, on some occasions, members of their own species. There is also a group that feeds on barnacles and occasionally anemones. Some nudibranchs rely on enzymes, rather than teeth, to break down prey. In turn they are also the prey of certain fish, sea spiders, turtles, sea stars, a few crabs.

They are well equipped to defend themselves however. Tough-skinned, bumpy and abrasive, some are able to camouflage, however, others have bright colouring making them highly visible. This warns they are distasteful or poisonous. Nudibranchs hoard capsules of tightly coiled stingers, called nematocysts, and automatically release a sour liquid from the skin when touched.

Nudibranchs have hardly given up all their secrets; scientists estimate they've identified only half of all nudibranch species, and even the known ones are elusive. Most live no more than a year and then disappear without a trace, their boneless, shell-less bodies leaving no record of their colourful lives.